1. Field of the Invention
This invention relates to a composite hydraulic system for providing two separate functions on a work vehicle such as causing steering of an articulated loader vehicle and operating the implements of the same articulated loader vehicle.
2. Description of the Prior Art
U.S. Pat. No. 2,353,388 Cannon and 2,353,389 Cannon disclose hydraulic systems which operate on a similar principle to the present invention. That is, they are three-pump systems in which two pumps respectively provide the principal supply to separate hydraulic circuits while the third pump is a variable displacement pump connected between the two circuits to provide transfer of hydraulic fluid in either direction at variable volume depending on external conditions. In these two patents all three pumps are driven by individual electric motors, and since they are always transferring fluid from low pressure to high pressure they are incapable of regeneration. However, in the present invention the three pumps are driven by the same engine thereby providing for regeneration.
U.S. Pat. No. 3,355,994 Malott is directed to solving the same problem solved by the present invention, namely, to improve the performance and efficiency of the steering and implement hydraulic circuits on a work vehicle. Malott uses a three-pump system with all three pumps driven by the same engine, however, all three pumps also operate from the same reservoir. In other words, the third pump is not a transfer or switch pump as in the present invention but instead supplies either or both of the two individual hydraulic circuits depending upon the condition of a flow dividing or demand valve. The condition of such valve in turn is dependent upon the flow in the steering system which is sensed by a variable orifice in the demand valve.
U.S. Pat. No. 3,535,877 Becker et al teaches the addition of an unloader valve to the hydraulic system of the Malott patent. The unloader valve diverts to the reservoir either the output of the third pump or the implement output of the demand valve when implement pressure exceeds a predetermined value, and delivers the output to the implement circuit when that pressure reduces to a predetermined lower value.